1. Field of the Invention
The present invention relates to an image forming apparatus that can form two images on one round of an intermediate transfer material.
2. Related Background Art
Early copying machines and color printers which output color documents in offices were not commonly used because of expensive main parts cost and running cost in comparison with monochrome machines, although there were potential demands. This is because most business documents were monochrome outputs and there were few color copying machines and color printers with low main parts costs and low running costs to meet the demand. However, color copying machines and color printers achieving main part costs and running costs almost equivalent to monochrome machines as office applications and enabling color outputs easily in offices have been developed in recent years. Hence, switching to the color machines instead of conventional monochrome machines has been progressing in offices.
In order to replace a monochrome machine with a color machine in this manner, a space of its main part also becomes important as well as the realization of functions being the same as those of a monochrome machine. For this reason, in comparison with a tandem type color image forming apparatus formed by horizontally arranging four photoconductive (or photosensitive) drums which form four colors of images for color image formation concurrently, a one-drum type image forming apparatus, which uses one photoconductive drum and transfers an image, formed on the photoconductive drum, on an intermediate transfer material, and forms four colors of images by four revolutions of a developer by switching the developer to another every round of the intermediate transfer material, can not only decrease the size of the apparatus itself, but also can keep the main part cost low. In addition, in the case of printing both sides, although it is necessary to reverse a sheet, on the one side of which an image is formed, and to convey the sheet to a position where the double-sided sheet is resupplied, it is possible to suppress the size of the apparatus by making a reversing port for reversing this sheet serve also as a sheet discharging port.
However, in such a one-drum type color image forming apparatus, since it is necessary to perform color image formation by the four rounds of the intermediate transfer material, the productivity of a color output is low in comparison with that of a tandem system.
Therefore, as shown in U.S. Pat. No. 6,204,927, for example, in regard to an image in A4 size or letter (LTR) size which is sheet size generally used in an office, the decrease of productivity is suppressed as much as possible by outputting two images at a time by performing two sheets of image formation on an intermediate transfer material while the intermediate transfer material, having the peripheral length enabling image formation in A3 size, takes one round (thereinafter, this is called “two-sheet affixing” or “two-affix”).
As described above, in color image formation, the two-sheet affixing is performed in principle to form two sheets of images so that they may exist concurrently.
However, in the two-sheet affixing, since two images on the intermediate transfer material are close, a sheet interval between two sheets on which these images should be transferred must be conveyed with approaching with each other. On the other hand, in the case of the double-sided image formation accompanied by reversal in a sheet discharging port, a sheet interval between the sheet, being reversed, and a sheet following the sheet being reversed should be such that the subsequent sheet may be conveyed to the sheet discharging port after the reversal operation of the sheet being reversed is completed. Therefore, if control is such that the two-sheet affixing is performed in the order of sheets being ready for image formation, there arises a problem that the reversal of the sheet in the sheet discharging port cannot be performed.
Nevertheless, if image formation is performed in a one-sheet affixing mode in all pages, there is a problem that productivity falls remarkably.